1. Field of the Invention
The present invention relates generally to an apparatus for receiving broadcasting signals, and more particularly, to a broadcasting signal receiving apparatus which is operative to receive not only an analog audio broadcasting signal, such as an amplitude-modulated (AM) audio broadcasting signal or a frequency-modulated (FM) audio broadcasting signal, but also a digital audio broadcasting (DAB) signal for obtaining a reproduced sound signal.
2. Description of the Prior Art
Although an analog audio broadcasting system which includes an AM audio broadcasting system in which audio information signals are transmitted in the form of an AM audio information signal and a FM audio broadcasting system in which audio information signals are transmitted in the form of a FM audio information signal, has been put to practical use for a long time in the field of audio broadcasting, there has been recently proposed to introduce a digital audio broadcasting system in which audio information signals are transmitted in the form of a digital audio information signal for the purpose of improving quality of audio information transmitted or received in the system. Especially, in the European Continent, the digital audio broadcasting system called "DAB" has been already put to practical use in some countries.
It is expected that the digital audio broadcasting system would have great development henceforth so as to be in the mainstream in the field of audio broadcasting, in place of the analog audio broadcasting system, some time in the not so far future. However, at present, in a region wherein the digital audio broadcasting system has been already put to practical use or has been concretely planed to be materialized, a service area in which the digital audio information signal transmitted from a broadcasting station can be properly received is restricted to be relatively small. Therefore, in the case where the digital audio broadcasting is actually carried out, the analog audio broadcasting is also carried out, in addition to the digital audio broadcasting, so that the same program is transmitted through each of the digital audio broadcasting and the analog audio broadcasting at the same time.
In the analog audio broadcasting, the analog audio information signal, such as the AM or FM audio information signal, is transmitted from an analog signal broadcasting station in the form of the analog audio broadcasting signal, and in the digital audio broadcasting, the digital audio information signal is transmitted from a digital signal broadcasting station in the form of the digital audio broadcasting signal. The digital audio broadcasting signal transmitted in the digital audio broadcasting contains audio information data which represents audio information. Such audio information data are usually subjected to a time interleaving arrangement for the purpose of minimizing deterioration resulting from data bit omission, data bit transformation and so on.
The audio information data are transmitted in the form of a series of unit segments each having a time duration of, for example, 24 ms and constituting a logical frame. The time interleaving arrangement to which the audio information data are subjected is carried out with an interleaving completion segment consisting of successive sixteen logical frames. Accordingly, time interleaved audio information data corresponding to each group of successive sixteen original logical frames constitute successive sixteen new logical frames. This means that the time interleaved audio information data are provided with a relatively long delay time, such as more than 400 ms, which is longer than a time corresponding to successive sixteen original logical frames. Consequently, it is understood that the audio information data contained in the digital audio broadcasting signal transmitted from the digital signal broadcasting station has been provided with the relatively long delay time of, for example, 400 ms.
Under such a situation as mentioned above, it has been proposed to use an analog/digital broadcasting signal receiver which is an apparatus for receiving both the analog audio broadcasting signal transmitted from the analog signal broadcasting station and the digital audio broadcasting signal transmitted from the digital signal broadcasting station in an area in which both of the analog audio broadcasting and the digital audio broadcasting are put into practice. The analog/digital broadcasting signal receiver comprises an analog audio broadcasting signal receiving portion for receiving the analog audio broadcasting signal and obtaining a reproduced sound signal based on the received analog audio broadcasting signal (a reproduced analog broadcasting sound signal), a digital audio broadcasting signal receiving portion for receiving the digital audio broadcasting signal and obtaining a reproduced sound signal based on the received digital audio broadcasting signal (a reproduced digital broadcasting sound signal), and a reproduced signal selecting portion for deriving selectively the reproduced analog broadcasting sound signal and the reproduced digital broadcasting sound signal.
In the digital audio broadcasting signal receiving portion of the analog/digital broadcasting signal receiver, the audio information data contained in the received digital audio broadcasting signal are subjected to various signal processings so that the reproduced digital broadcasting sound signal is obtained. One of these signal processings to which the audio information data contained in the received digital audio broadcasting signal are subjected is a time de-interleaving arrangement for releasing the audio information data from the time interleaving arrangement to revert to original audio information data. The time de-interleaving arrangement to which the audio information data contained in the received digital audio broadcasting signal are subjected is carried out to each group of successive sixteen logical frames constituting the interleaving completion segment. That is, sixteen logical frames are successively derived from each of the interleaving completion segments of the audio information data contained in the received digital audio broadcasting signal to be stored in a memory device, and after those sixteen logical frames are once stored in the memory device, each of them is read from the memory device in a predetermined deinterleaving manner so that the time de-interleaving arrangement is carried out to each of the interleaving completion segments of the audio information data contained in the received digital audio broadcasting signal.
Meanwhile, in the analog audio broadcasting signal receiving portion of the analog/digital broadcasting signal receiver, the received analog audio broadcasting signal is subjected to a demodulation processing and other signal processings, for example, a frequency-demodulation processing, a de-emphasis processing and so on in case of the FM audio broadcasting signal, so that the reproduced analog broadcasting sound signal is obtained.
Then, the reproduced digital broadcasting sound signal obtained from the digital audio broadcasting signal receiving portion and the reproduced analog broadcasting sound signal obtained from the analog audio broadcasting signal receiving portion are selectively derived through the reproduced signal selecting portion in such a manner that the reproduced digital broadcasting sound signal is selected when it is obtained appropriately and the reproduced analog broadcasting sound signal is selected when the reproduced digital broadcasting sound signal is not obtained appropriately.
In the analog/digital broadcasting signal receiver as described above, since the time de-interleaving arrangement is carried out to each group of successive sixteen logical frames constituting the interleaving completion segment of the audio information data contained in the received digital audio broadcasting signal in the digital audio broadcasting signal receiving portion, the time de-interleaved audio information data obtained in the digital audio broadcasting signal receiving portion are provided with a delay time which is longer than a time corresponding to the group of successive sixteen logical frame. Consequently, the reproduced digital broadcasting sound signal obtained based on the audio information data contained in the received digital audio broadcasting signal from the digital audio broadcasting signal receiving portion has a relatively long delay time, such as more than 400 ms, which has been caused mainly by the time de-interleaving arrangement carried out in the digital audio broadcasting signal receiving portion.
On the other hand, the reproduced analog broadcasting sound signal which is obtained based on the received digital audio broadcasting signal subjected to the demodulation processing and other signal processings, for example, the frequency-demodulation processing, the de-emphasis processing and so on in case of the FM audio broadcasting signal, from the analog audio broadcasting signal receiving portion has a relatively short delay time, such as less than several tens .mu.s, which has been caused mainly by the signal processings carried out in the analog audio broadcasting signal receiving portion.
Accordingly, when the digital audio broadcasting signal transmitted from the digital signal broadcasting station and the analog audio broadcasting signal transmitted from the analog signal broadcasting station are received respectively by the digital audio broadcasting signal receiving portion and the analog audio broadcasting signal receiving portion of the analog/digital broadcasting signal receiver under a situation in which the same program is transmitted from each of the digital signal broadcasting station and the analog signal broadcasting station at the same time, the reproduced digital broadcasting sound signal and the reproduced analog broadcasting sound signal, each of which represents the same sound information, are obtained with a relatively large time difference between each other from the digital audio broadcasting signal receiving portion and the analog audio broadcasting signal receiving portion, respectively. That is, first, the reproduced analog broadcasting sound signal with a relatively short delay time is obtained from the analog audio broadcasting signal receiving portion, and then, the reproduced digital broadcasting sound signal with a relatively long delay time, which represents the same audio information as that represented by the reproduced analog broadcasting sound signal, is obtained from the digital audio broadcasting signal receiving portion.
In such a case, the relatively long delay time with which the reproduced digital broadcasting sound signal has as explained above results in the sum of the relatively long delay time, such as 400 ms, which the audio information data contained in the digital audio broadcasting signal has been provided when it is received by the digital audio broadcasting signal receiving portion and another relatively long delay time, such as 400 ms, which is caused in the digital audio broadcasting signal receiving portion, and therefore, reaches to, for example, 800 ms.
The above mentioned operation by which the reproduced digital broadcasting sound signal and the reproduced analog broadcasting sound signal each representing the same sound information are obtained with the relatively large time difference between each other from the digital audio broadcasting signal receiving portion and the analog audio broadcasting signal receiving portion, respectively, brings disadvantages on the analog/digital broadcasting signal receiver as described below.
When the analog/digital broadcasting signal receiver is equipped to a vehicle and moved with the vehicle from a position in a condition wherein the reproduced analog broadcasting sound signal is obtained appropriately from the digital audio broadcasting signal receiving portion to another position in a condition wherein the reproduced analog broadcasting sound signal is not obtained appropriately from the digital audio broadcasting signal receiving portion and thereby the signal selection by the reproduced signal selecting portion is changed from the reproduced digital broadcasting sound signal to the reproduced analog broadcasting sound signal, a discontinuation of signal resulting from the relatively large time difference is caused between the reproduced digital broadcasting sound signal which had been derived before the change in the signal selection by the reproduced signal selecting portion and the reproduced analog broadcasting sound signal which is derived after the change in the signal selection by the reproduced signal selecting portion. This results in deterioration in quality of the reproduced sound signals obtained from the analog/digital broadcasting signal receiver.
Such a discontinuation of signal between the reproduced digital broadcasting sound signal and the reproduced analog broadcasting sound signal is also caused when the signal selection by the reproduced signal selecting portion is changed from the reproduced analog broadcasting sound signal to the reproduced digital broadcasting sound signal.